Scientific References

Peer Reviewed Literature: A combustion setup to precisely reference 13C and 2H isotope ratios of pure CH4 to produce isotope reference gases of 13C-CH4 in synthetic air

Authors Sperlich P.; Guillevic, M. ; Buizert, C. ;Jenk, T. M. ; Sapart, C. J. ;
Presented at
Atmospheric Measurement Techniques http://dx.doi.org/10.5194/amt-5-2227-2012

Abstract

Isotope records of atmospheric CH4 can be used to infer changes in the biochemistry of CH4. One factor limiting quantitative estimates of changes in the biogeochemistry of CH4 are the uncertainties of the isotope measurements due to the lack of a unique isotope reference gas, certified for 13C-CH4 or 2 5 H-CH4. We present a method to produce isotope reference gases for CH4 in synthetic airs that are precisely anchored to the VPDB and VSMOW scale and contain 13C-CH4 values typical for the modern and glacial atmosphere. We quantitatively combusted two pure CH4 gases from fossil and biogenic sources and determined the 13C and 2H values of the produced CO2 10 and H2O relative to the VPDB and VSMOW scale within a very small analytical uncertainty of 0.04‰ and 0.7 ‰, respectively. We found isotope ratios of −39.56‰ and −56.37‰for 13C and −170.1‰and −317.4‰for 2H in the fossil and biogenic CH4, respectively. We used both CH4 types as parental gases from which we mixed two filial CH4 gases. Their 13C was determined to be −42.21‰ and −47.25 ‰, representing 15 glacial and present atmospheric 13C-CH4. The 2H isotope ratios of the filial CH4 gases were found with −193.1‰ and −237.1 ‰, respectively. Next, we mixed aliquots of the filial CH4 gases with ultrapure N2/O2 (CH4 _ 2ppb) producing two isotope reference gases of synthetic air with CH4 mixing ratios near atmospheric values. We show that our method is reproducible and does not introduce isotopic fractionation for 13C 20 within the uncertainties of our detection limit (we cannot conclude this for 2H because our system is currently not prepared for 2H-CH4 measurements in air samples). The general principle of our method can be applied to produce synthetic isotope reference gases targeting 2H-CH4 or other gas species.